As pointed out by the NIH (National Institutes of Health) in a recent RFA (Request for Applications) “3.8 million Americans are transfused with 28.2 million blood components derived from 12.8 million units of blood donated by apparently healthy volunteers.” Blood products are extensively tested for the presence of pathogens prior to administration in the developed world. Nevertheless, there exists a small, but finite risk of transmission of infectious agents in a transfusion. In its RFA, the NIH estimated the risk during transfusion “of a unit of screened blood is 1/1,000,000 for hepatitis A virus (HAV), is 1/30,000-1/50,000 for hepatitis B (HBV), 1/30,000-1/150,000 for hepatitis C (HCV), 1/20,000-1/2,000,000 for human immunodeficiency virus (HIV), 1/250,000-1/2,000,000 for human T-cell lymphotropic viruses (HTLV) types I and II and 1/10,000 for parvo virus B1. Furthermore, the estimated frequency of bacterial contamination of red blood cells is 1/500,000 units and the risk of platelet-related sepsis is estimated to be 1/12,000” as of the year 2000.
The risks of viral infection are due to the “window period,” the period of time between the infection of a donor and the development of detectable levels of antibodies. The introduction of nucleic acid testing (NAT) has shortened the window period and further decreased the incidence of units of blood products containing pathogens. NAT was introduced for HIV and HCV in the United States in 1998. This new technology has greatly reduced the risk as the window period has been dramatically decreased. The window period for HIV was reduced from 22 days to 11 days and the window period for HCV has been reduced from 70 days to 8-10 days. This new technology has brought the risks per unit of donated blood to 1:1,800,000 for HIV and 1:1,600,000 for HCV. When these statistics are compared to the current risk of 1:220,000 associated with HVB, a virus for which NAT is not currently performed, the need for additional pathogen reduction technologies is apparent. As the blood supply in the developed world is relatively safe, any technology that further decreases the incidence of pathogens in blood products must itself be incredibly safe to be of net benefit to public health.
Sadly, blood products are still not screened prior to administration in many parts of the world. In underdeveloped countries, the cost of introducing testing protocols exceeds the local resources. The blood supply in too many countries tragically contains high levels of pathogens, especially parasitic organisms such as those responsible for Chagas Disease and Malaria. New technology which can improve the safety of the blood supply in underdeveloped portions of the globe must be simple and inexpensive if it is to receive widespread use.
A simple technology which has been used to sterilize at least some portions of the blood such as plasma is UV light. However, pathogens are composed of the same amino acid, nucleic acid, and lipid building blocks as plasma proteins, platelets, and red cells. Consequently, there is no known wavelength of light which can be selectively deposited into pathogens in the presence of blood products. Pathogens, plasma proteins, and platelets absorb UVB (280-320 nm) and UVC (200-280 nm) radiation. This inactivates pathogens, but with unacceptable damage to plasma proteins and platelets. Red cells absorb this type of radiation so strongly that one cannot inactivate pathogens with UVB and UVC radiation in their presence. UVA radiation (320-400 nm) alone does not inactivate a virus. It does, however, shorten the shelf life of platelets.
This fact has led practitioners to study sensitizers. By definition, sensitizers absorb light and initiate chemical reactions that inactivate pathogens. Thus, it is no surprise that sensitizers themselves undergo chemical change upon photolysis. Therefore, not only must the ideal sensitizer be innocuous, but every molecule it degrades to upon photolysis must be non-toxic and non-mutagenic as well.
Ideally, a sensitizer will absorb long wavelength light (λ>400 nm) that is not absorbed by plasma proteins or platelets. If the sensitizer binds only to pathogen, in the presence of plasma protein and platelets, radiant energy in the UVA or visible region of the spectrum can be deposited selectively into the pathogen, even in the presence of blood components. This can lead to inactivation of the pathogen, in principle, by various chemical reaction mechanisms.
In the absence of pathogen, the ideal sensitizer will not bind to transfusable blood product. Thus the ideal sensitizer must recognize and exploit a chemical difference between the pathogen and the blood product. There is a chemical difference between many pathogens and plasma proteins. Bacteria and many types of virus are encapsulated by phospholipid membranes or envelopes, but plasma proteins are not. Pooled plasma is treated with solvent detergent that dissolves membranes and inactivates many types of pathogens without damage to the plasma protein. Recently, Cerus Corporation has developed a technology for pathogen reduction of platelets that has been approved for use in Europe. At this time there is no pathogen eradication technology used to treat red cells before their administration in transfusion medicine, anywhere in the world.
A successful photochemical technology that will eradicate pathogens present in blood products requires that the perfect sensitizer has the following properties: the ideal sensitizer must bind to pathogenic particles but must not bind to plasma proteins, platelets or red blood cells. Secondly, the ideal sensitizer should be non-toxic, non-mutagenic and must not break down to compounds that are toxic and mutagenic. It should be readily available, water soluble, and inexpensive. Finally, the ideal sensitizer must absorb UV and/or visible radiation. Absorption of radiation must produce a short-lived, highly-reactive toxin, which creates lesions in its immediate vicinity (e.g., only to the pathogenic particle to which it is bound).
Molecules that are currently under investigation, which fulfill many of the above-mentioned requirements, have flavin moieties and are members of the alloxazine family, in particular, riboflavin.